In RFID systems that have been in increasing use recently, an antenna for information communication is mounted in each of a mobile communication terminal and a reader/writer, so that data is exchanged between the mobile communication terminal and the reader/writer. In particular, there is a high demand for an antenna mounted in a mobile electronic device to achieve high performance, low cost, and small size. Magnetic antennas that have a magnetic core and achieve such a demand are disclosed in Japanese Patent No. 3957000 (Patent Document 1), Japanese Unexamined Patent Application Publication No. 2002-325013 (Patent Document 2), and Japanese Unexamined Patent Application Publication No. 2002-298095 (Patent Document 3).
FIG. 1A is a perspective view of a magnetic antenna disclosed in Patent Document 1, and FIG. 1B is a plan view of the magnetic antenna. In an antenna device 23 shown in FIGS. 1A and 1B, an antenna coil 22 is mounted on a circuit board 21. On the circuit board 21, the antenna coil 22 is mounted such that lateral edges of the antenna coil 22 coincide with edges of the circuit board 21 in a crosswise direction thereof.
The antenna coil 22 has a first magnetic core 24a, a second magnetic core 24b that is provided so as to be spaced apart from the first magnetic core 24a, and first and second coil portions 22a and 22b that are coaxially wound on the first and second magnetic core 24a and 24b, respectively, in opposite directions.
Patent Document 2 discloses a configuration in which an air-core coil formed on a plane to have a substantially scroll shape, and a plate-like core member inserted into the air-core coil, are included, and the core member is provided so as to be substantially parallel with the plane of the air-core coil.
Patent Document 3 discloses a configuration in which a magnetic body is provided under a loop coil.
In the antenna device disclosed in Patent Document 1, when the interval between the circuit board 21 and the antenna coil 22 increases, the rate of a magnetic flux penetrating in the coil axis direction decreases. Thus, a magnetic field linked with the coil decreases, resulting in deterioration of the coupling coefficient.
In the antenna device disclosed in Patent Document 2, only a magnetic flux passing on one side of a metal plate is linked with the coil. Thus, sufficient communication performance is not obtained.
In the antenna device disclosed in Patent Document 3, when there is a conductor plate under the antenna, the self-inductance of a coil that does not contribute to coupling increases, resulting in deterioration of the coupling coefficient.